This invention relates to a method of manufacturing or repairing gas turbine components in which cooling holes have been partially or fully obstructed with coating material by laser drilling to open such cooling holes.
Advanced gas turbine engine components operating in the hot sections of the engine can experience temperatures which can contribute to reduction in the operating life of the components. Generally, such components include turbine blades and vanes, combustors, turbine shrouds and various components in the exhaust system. To improve component life, such components have been designed to include cooling passages within the body of the component exiting a surface or about a component surface. For additional thermal and or environmental resistance, frequently there is included on a surface exposed to higher temperatures, a high temperature protective coating such as a metallic or ceramic type coating. Examples of such coatings include aluminides, platinum aluminides, MCrAlY (in which M is at least one of the elements Fe, Co, and Ni), various metal overlay type coatings, and ceramic thermal protective coatings, one type of which is referred to in the art as Thermal Barrier Coatings or TBC. Typical of TBC coatings is one based on zirconia stabilized with yttria, for example about 92 wt. % zirconia stabilized with about 8 wt. % yttria. Methods for application of a TBC coating include plasma spraying and electron beam physical vapor deposition. Frequently, such coatings are used with a metallic bond coat.
In order to function properly, the cooling holes must be constructed to a specified configuration and dimensions because the distribution of air flow must be controlled to achieve proper cooling of the component during engine operation. Thus, the cooling holes must not be blocked or even partially blocked to provide a sufficient and uniform cooling air distribution through the component interior and across the exterior of the component. Application of a coating can result in a significant reduction in air flow through cooling holes and can result in complete closure of the cooling holes.
Therefore, after a coating is applied the cooling holes may have to be opened to their original specified dimensions to provide proper cooling of the component and/or to restore proper airflow and the holes must be opened without causing damage to the parent material of the component or to the coating in areas other than where it is desired to remove the coating. A method of opening the holes needs to provide precise control to redrill into an existing cooling hole as well as a process which can be operated efficiently on a commercial basis with regard to reduced process time and avoidance of errors in drilling.
While the specifications (e.g. blueprint) for a component will indicate the general location of the cooling holes, minor variations will occur during manufacturing when the cooling holes are installed (e.g. by laser drilling, EDM, ECM or the like), with these minor variations becoming critical when redrilling is attempted. Further, with components being repaired variations in cooling hole location can occur due to use or repair operations. Thus precise and accurate control of the laser drilling to reopen the cooling holes is required. In addition, during laser drilling a significant amount of debris is generated which can interfere with further drilling, particularly when a machine vision system is utilized, requiring frequent stopping and cleaning of the component.
The use of an Excimer laser for opening a blocked cooling hole is described in U.S. Pat. No. 5,216,808; however, a process which can be efficiently operated utilizing a ND/YAG type laser is desired.
Briefly, the present invention provides a process for laser drilling to remove coating material from obstructed cooling holes of a gas turbine component by utilizing a CNC component program which is preprogrammed with the general location of the cooling holes of a component to move a machine vision system and component relative to each other to each cooling hole location for a series of cooling holes, determining the actual location for each cooling hole and storing in the memory of the CNC or a data storage device the actual locations for the cooling holes for the series of cooling holes, followed by laser drilling based on the actual location of the cooling holes stored in the memory of the CNC or the data storage device to remove the coating material obstructing the cooling holes for the series of cooling holes. This process can be advantageously carried out to determine the actual location of the cooling holes either on a coated component or a component that has been prepared for coating.